Field of the Invention
The invention relates to processes in which a dense fluid is injected into a subterranean formation, and particularly to a method for generating electricity utilizing the excess downhole pressure of a fluid flowing through an injection well and into a subterranean formation
Description of the Prior Art
There are a wide variety of injection operations in which a dense fluid is injected into the earth, including the disposal of liquid waste streams from urban or industrial plants, the reinjection of effluent liquids from a geothermal power plant, the injection of displacement fluids in the various enhanced oil recovery processes wherein the injected fluid is employed to displace oil and/or gas from a subterranean formation, and the injection of leaching solutions into a subterranean formation to recover mineral values therefrom. Operations which require fluid injection, or in which fluid injection is desirable, have at least one well communicating between the surface location from which the fluid is to be injected and the subterranean formation into which the fluid is to be injected. These wells vary in depth from several feet to several thousand feet and more, depending of course upon the location of the particular subterranean formation relative to the surface location. For example, wells employed in enhanced oil recovery methods vary in depth from less than about 500 feet to 25,000 feet or more, with wells on the order of 2,000 to 10,000 feet being common.
The subterranean formations into which the fluids are injected contain a naturally occurring fluid and/or a previously injected fluid, herein collectively referred to as the "connate fluid", which exerts a back pressure, herein called the "connate fluid pressure", in resistance to the injection operation. This resistance must be overcome by some positive pressure supplied in the injection operation. In some cases, the positive pressure is supplied by external pumps or compressors; however in many cases, external devices are not required because the hydrostatic head of the fluid column in the injection well inherently formed in the operation provides sufficient positive pressure to force the fluid into the formation. In these latter cases, an equilibrium fluid level, herein called the "dynamic level", will be established in the well at a point between the earth surface and the formation. The location of the dynamic level will depend, inter alia, upon the connate fluid pressure and the fluid injection rate. Above the dynamic level there exists a relatively empty portion of the well which, if it were to be filled with the fluid, would provide additional hydrostatic head. Thus, in these injection operations there is available a hydrostatic head in excess of that required to force the fluid into the formation at the desired rate. The method of this invention utilizes this available, but previously unused, excess hydrostatic head to generate electrical energy.
One problem with the injection of fluids into relatively pressure-sensitive formations, such as unconsolidated formations, is that the high downhole pressures inherently produced in some fluid injection operations may result in a reduction in well injectivity. Reduced well injectivity, of course, requires a higher pressure for the same fluid flow rate which higher pressure results in a further reduction in well injectivity. Such operations would benefit from a method of controlling the downhole pressure during the injection operation.
A method for downhole pressure control would also benefit some of the enhanced oil recovery processes in which a displacement fluid comprised of a highly pressurized, normally gaseous fluid is injected into an oil-bearing formation to miscibly displace the oil through the formation to a production well. To achieve miscible-displacement, the normally gaseous fluid must be highly compressed to form a dense fluid, and is therefore introduced into the top of the injection well at an elevated pressure. The fluid pressure adjacent the formation in the prior art injection method would then be equal to this high surface pressure plus the hydrostatic head of the dense fluid column in the well. Where this downhole pressure is in excess of the desired formation flooding pressure, as will occur in relatively deep wells, the compression requirement of the operation will be increased. Thus, there exists a need for controlling the excess downhole fluid pressure in these injection operations.
Accordingly, it is a primary object of this invention to utilize the excess potential energy available during the injection of a dense fluid through an injection well and into a subterranean formation, without adversely affecting the injection operation itself.
Another object of the invention is to utilize the available excess hydrostatic head of a dense fluid flowing through an injection well and into a subterranean formation to generate electrical energy.
Still another object of this invention is to provide an improved method for controlling the flow of a dense fluid through an injection well and into a subterranean formation, in which the available excess hydrostatic head of injection fluid in the well is utilized for the generation of electricity.
Yet another object of this invention is to provide a method for controlling the downhole pressure exerted on a subterranean formation during the injection of a dense fluid through an injection well and into the formation, in which the excess pressure is utilized for the generation of electricity.
Further objects, advantages and features of the invention will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings.